It all depends on your background. If you have already used any MPC before its cake. If you have used other sequencers and samplers, you'll pick it up pretty quick once you learn the MPC architecture. With no experience, it takes a while to grasp how everything works and some of the basic audio concepts like sequencing, sampling, envelopes, filters, etc.
The XL was my first piece of hardware. I took that manual to the bathroom, parties, airplanes, church, watever and read it many times until I understood everything. Once you know the manual, there are a few tips and tricks (most you can find here) and then you should know 99% of its capabilities. Its really not that deep of a machine when you compare it to the 4k or some of the software sequencers out there now. I'd say it took me a year to know it in and out (painted over the button lables), and another year to get even better and learn all the tricks. But its all about how much time you spend with it.